Previous Generations of RTGs


Power source for Nimbus III meteorological satellite. SNAP stands for "Systems for Nuclear Auxilliary Power."


  • Output 28.2 Watts electric (or We) at beginning of mission
  • NASA's first application of radioisotope power systems
  • Nimbus B-1 launch on 18 May 1968
    • Launch vehicle failure forced destruction by range safety officer
    • Spacecraft and upper stage sank in Santa Barbara Channel
    • RTGs recovered and fuel reused for Nimbus III

Viking SNAP 19
Cutaway view of a SNAP-19 RTG used to power NASA's two Viking landers


Power source for Viking 1 & 2 Mars landers, Pioneer 10 & 11 spacecraft.


  • Output 40.3 Watts electric (Pioneer) and 42.6 Watts electric (Viking) at beginning of mission
  • Modified version of SNAP-19B
  • Pioneer 10 & 11 design lifetime was 5 years; spacecraft continued to communicate with Earth for 30 and 22 years respectively.
  • Viking 1 & 2 operational requirement was 90 days; landers operated for six and four years respectively.

Apollo astronaut photo of a SNAP-27 RTG on the Moon


Power source for Apollo Lunar Surface Experiment Package (ALSEP). Deployed on Apollo missions 12, 14, 15, 16 and 17.


  • Output 70 Watts electric at beginning of mission
  • Two-year design lifetime. All deployed units operated five to eight years until ALSEP stations were shut down.

Multi-Hundred Watt (MHW) RTG

Cutaway view of a Multi-Hundred Watt RTG

Power source for Voyager 1 & 2 spacecraft.


  • Output 158 Watts electric at beginning of mission
  • RTGs still operating over 40 years later at edge of solar system

General Purpose Heat Source (GPHS) RTG

GPHS cutaway
Cutaway view of a General Purpose Heat Source RTG

Power source for Galileo, Cassini, Ulysses & New Horizons spacecraft.


  • Output 292 Watts electric at beginning of mission
  • A total of 18 general purpose heat source (GPHS) modules are stacked together to provide the heat source for each GPHS RTG.
  • New Horizons RTG is still operating and enabled an extended mission deeper into the Kuiper Belt beyond Pluto.
  • Galileo operated for nearly 14 years, while Ulysses and Cassini functioned for nearly 19 years in space.

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